The present invention relates to form, fill and seal machines and is concerned in particular with the method and apparatus for sealing thermoplastic or other heat sealable materials which are formed into packages in the machine.
The process of forming heavy duty or industrial size bags in form, fill and seal machines is disclosed in U.S. Pat. Nos. 3,925,963 and 4,109,792. The bags are generally made for twenty-five pounds (11.35 kgs) or more of a product which may be a loose, powdered or granular material such as fertilizer, grain, coal or other bulk materials. The packaging material is supplied to the machine as a web and is formed into a tube which is closed at one end and into which the product is deposited in measured amounts. One bag is formed and filled while the previously formed bag is closed, sealed and severed from the tube by transverse sealing heads.
Because of the size and weight of the bags and also the types of material contained in the bags, the thermoplastic material used is generally a heavy gauge material having, for example, up to a 0.010 inch single or double ply thickness. In the course of a bag-forming operation in the machine, several plies of the material are placed in overlapping relationship to form the long or longitudinal seal and the transverse seals at top and bottom ends of a bag. Thus, the seals must be formed through multiple plies of the bag material. Bags with gussets for stable stacking may require that welds in the sealing areas penetrate through 0.020 inch of the material in some locations and twice that thickness in other areas. For this reason, the machines disclosed in the referenced patents utilize hot gas sealing heads which apply both heat and uniform pressure to one side of the multi-ply sealing areas. The resulting welds are without undesirable thinning due to the uniform pressure and unrestrained shrinkage. However, the application of heat to just one side of the sealing area limits the rate of heating to the fusing temperature, and correspondingly, the cyclic rate of the machine.
It is, accordingly, a general object of the present invention to provide a sealing mechanism that accommodates a wide range of material thicknesses at high packaging rates without decreasing seal integrity.